This invention relates to ultrasonics and, more particularly, to the generation and detection of ultrasonic waves in materials.
Precision measurements useful for evaluating elastic properties of the surface of a material are difficult to obtain. Small changes in the physical properties of a medium, however, have been precisely measured by sensing a relative shift in the phase or amplitude of two orthogonally polarized waves which occurs when such waves interact with the medium. In an optical ellipsometer, for example, a series of polarizers and quarter wave plates are utilized to measure a change in the elliptical polarization of an optical beam, which is caused as a result of the reflection of the beam from a metallic surface. Such a change occurs because the orthogonal components of the light beam, which are polarized parallel and perpendicular to the plane of incidence, exhibit identical propagation properties in a medium such as air, but are affected differently upon reflection by such a surface. Using this technique to measure such changes, foreign layers as thin as one atomic thickness, 10.sup.-3 .mu.m or less, can readily be detected on a reflecting metal surface.
In an isotropic solid, ultrasonic shear elastic waves conceivably could be employed in an analogous manner to sense surface properties of the material, since ultrasonic waves will propagate in a solid with two different polarizations having equal propagation velocities. Three major difficulties, however, have in the past prevented the application of this technique to ultrasonic waves travelling in solid media. First, longitudinal as well as transverse ultrasonic waves will propagate in a solid. Second, in contrast to the optical isotropy of air, most polycrystalline solids contain small anisotropies which can cause the two ultrasonic waves to travel with different velocities. Measurements of shear wave birefringence, for example, are well known and have even been used to detect body stresses in solid materials. Consequently, the effects of bulk as well as surface properties of the test material are represented in known elastic wave techniques. Finally, elastic wave generating and detecting devices analogous to optical polarizer--quarter wave plate combinations have not heretofore been available in the art. Thus, piezoelectric transducers, for example, which are typically used for generating such waves, can be used to excite or detect ultrasonic waves having a fixed polarization, but the plane of the piezoelectrically generated wave polarization cannot be altered without physically rotating the piezoelectric transducer and reestablishing the mechanical bond necessary with such a system.
Therefore, a need has developed in the art for an ultrasonic transducer capable of generating an elliptically polarized wave whose polarization may be readily and conveniently adjusted.
Furthermore, it would be advantageous to provide a transducer which avoids the difficulties associated with the generation of longitudinal waves and with the birefringence effects caused by anisotropies in solids.